In order to reproduce metallic luster on printed matter, there has hitherto been used a print ink containing a pigment comprising a metal such as aluminum, metal, silver or brass or an alloy thereof, or containing a pearl pigment, or used a transfer foil by an adhesive or heat fusion.
Further, by recent development of ink jet printing technology, a demand for obtaining printed matter having metallic luster has increased also in ink jet printing.
On the other hand, aluminum pigments presently commercially available as use for metallic pigments have an average particle size of 10 μm or more (for example, see patent documents 1 and 2), so that it is extremely difficult to allow them to pass through fine nozzles (30 μm or less in diameter) of a printer or a filtering filter. Further, these aluminum pigments have a disadvantage that it is difficult to obtain specular gloss because of their excessively large particle size. In addition, the aluminum pigments come to react with the surrounding moisture when reduced in particle size. It is therefore extremely difficult to reduce the particle size to 1 μm or less by a conventional pulverizing method or atomizing method. Accordingly, a pigment having an average particle size of 200 nm or less, which is ordinarily used in an ink jet ink, can not be prepared.
On the other hand, there is a method of forming a thin metal film on a base material by using a noble metal colloid of gold, silver or the like. However, metal colloidal particles having an average particle size of tens of nanometers can not reproduce metallic luster. For example, stable gold colloidal particles have a particle size of about 10 to 20 nm, and the color of a colloid dispersion is purple in this state. In this case, in order to obtain a thin film of gold, it is necessary to heat at 150° C. or more. Further, it is also possible to stably disperse a gold colloid having a larger particle size by adding a protective colloid. However, in this case, no thin metal film is obtained unless treatment for decomposing and removing the protective colloid is conducted. From these reasons, in the method in which the metal colloid is used, after-treatment of some kind becomes necessary after printing. In particular, when heat treatment is conducted, there is a disadvantage that a print medium is extremely limited. Also when pressure treatment is conducted, there naturally occur a disadvantage that a print medium is limited and an equipment load of requiring pressure equipment (for example, see patent document 3). Further, although noble metals (Au, Ag, Ru, Rh, Pd, Os, Ir and Pt) are used as the metal colloids, it is difficult to use them in decoration, ornament, advertising printing and the like in terms of cost.
[Patent Document 1] U.S. Pat. No. 4,233,195
[Patent Document 2] U.S. Pat. No. 5,662,738
[Patent Document 3] JP-A-2004-175832